The present invention relates to a stripe buried semiconductor laser with current confinement structure.
A stripe buried semiconductor lasers are provided with a superior optical waveguide function as their active layer region is surrounded by substances having a lower refractive index. As the difference in refractive indices is set at a value higher than necessary in conventional stripe structure, oscillation in fundamental mode is effected when the stripe is less than 2 microns in width, but as the stripe becomes wider, the oscillation occurs in a higher order mode. The fundamental mode oscillation with such a narrow stripe inevitably limits its output typically to several mW or less.
An improved stripe buried semiconductor laser has been proposed to overcome the problem of low output encountered in the convenitonal buried semiconductor laser. In such a stripe buried structure, as will be described hereinafter, there is provided an optical waveguide layer in addition to the active layer. Semiconductor layers of lower refractive index surrounding the active layer completely confines injected carriers, and light is propagated in an optical waveguide layer which has a smaller refractive index than that of the active layer, thereby preventing oscillation in a higher order mode while maintaining single mode oscillation over a high output power range (See, for example, T. Kajimura et al., "(GaAl)As High Power Visible Lasers with Self-aligned Stripe Buried Heterostructure", Fourth International Conference on Integrated Optics and Optical Fiber Communication, June 27-30, 1983, Tokyo, Technical Digest, pp. 136-137.).
One example of prior art structure of the stripe buried semiconductor laser is shown in FIG. 1. In FIG. 1, the reference numeral 1 denotes an n-GaAs substrate, 2 an n-Al.sub.x Ga.sub.l-x As cladding layer (0.3.ltoreq.x.ltoreq.0.35), 3 an n-Al.sub.y -Ga.sub.l-y As optical waveguide layer (0.23.ltoreq.y.ltoreq.0.26), 4 an Al.sub.0.14 Ga.sub.0.86 As active layer, 5 a p-Al.sub.w Ga.sub.l-w As cladding layer (0.4.ltoreq.w.ltoreq.0.45), 6 a p-Al.sub.0.2 Ga.sub.0.8 As electrode layer, 7 a p-Al.sub.0.4 Ga.sub.0.6 As burying layer, 8 an n-Al.sub.0.4 Ga.sub.0.6 As burying layer, 9 a p-electrode and 10 an n-electrode.
In such a structure, the application of a forward voltage between the p-electrode 9 and the n-electrode 10 injects a current into the Al.sub.0.14 Ga.sub.0.86 As active layer 4 to enable laser operations. As the p-Al.sub.0.4 Ga.sub.0.6 As burying layer 7 functions as a current confining layer, the current flowing into regions other than the mesa region can be effectively prevented, allowing efficient injection of current into the mesa and improving the laser oscillation efficiency. The n-Al.sub.y Ga.sub.l-y As optical waveguide layer 3 allows laser light to propagate therein. As a result, the effective difference in the refractive index in the transverse direction of the active region 4 becomes small enough to prevent higher order mode oscillation. In other words, the effect of confining the light is reduced and stabilized oscillation in fundamental mode can be obtained over a wider range of output power even with a laser having a wider stripe.
However, in a semiconductor laser of such a structure, in view of reliability, the active layer 4 must be kept at a distance from the ohmic layer of the p-electrode 9. This inevitably necessitates the p-Al.sub.w Ga.sub.l-w As cladding layer 5 to have a greater thickness. Further, the specific resistivity of the cladding layer 5 is greater because of relatively greater Al composition(w), and coupled with its greater thickness, the thermal resistance of the cladding layer becomes greater, hampering high output power oscillation. Another disadvantage is that such a thickness of the p-Al.sub.w Ga.sub.l-w As layer 5 having a greater Al composition provides a greater opportunity for its side surface to be oxidized prior to the second liquid phase epitaxial (LPE) crystal growth process, because the layer 5 is exposed to the air. The oxidized film on the surface hinders the growth, greatly deteriorating the uniformity and reproducibility of the second LPE crystal growth. In other words, it becomes difficult to uniformly grow the burying layers 7 and 8 on the side surface of the mesa. If, however, the p-Al.sub.w Ga.sub.l-w As layer 5 is made thinner and the p-Al.sub.0.2 Ga.sub.0.8 As electrode layer 6 thicker, the boundary area between the p-Al.sub.0.2 Ga.sub.0.8 As electrode layer 6 and the n-Al.sub.0.4 Ga.sub.0.6 As burying layer 8 increases, whereby the p-n junction potential between the two layers becomes small and the leakage current flowing in the regions other than the mesa region increases.